In recent years, a technique to form a thin film transistor (hereinafter referred to as a TFT) over a substrate has made great progress and application to an active matrix display device has been advanced. In particular, a TFT formed using a poly-crystalline semiconductor film is superior in field-effect mobility to a TFT formed using a conventional amorphous semiconductor film; therefore, high-speed operation has become possible. For this reason, it is tried that a pixel, which has been controlled by a driver circuit provided outside a substrate, is controlled by a driver circuit formed over the same substrate as the pixel.
A substrate used in a semiconductor device is expected to be a glass substrate rather than a quartz substrate or a single-crystal semiconductor substrate in terms of cost. However, the glass substrate is inferior in heat resistance and easy to be deformed due to the heat. Therefore, when the TFT using the poly-crystalline semiconductor film is formed over the glass substrate, a laser irradiation method (referred to as laser annealing) is employed to crystallize a semiconductor film in order to prevent the glass substrate from being deformed due to the heat.
Compared with another annealing method which uses radiant heat or conductive heat, the laser annealing has advantages in that the processing time can be shortened drastically and that a semiconductor substrate or a semiconductor film over a substrate can be heated selectively and locally so that almost no thermal damage is given to the substrate. The laser annealing method described here indicates a technique to recrystallize an amorphous layer or a damaged layer formed in a semiconductor substrate or a semiconductor film, and a technique to crystallize a non-single crystal semiconductor film formed over a substrate. Further, a technique applied to planarization or modification of the surface of a semiconductor substrate or a semiconductor film is also included.
Laser oscillators used for the laser annealing can be broadly divided into two categories: pulsed laser oscillators and continuous wave (CW) laser oscillators according to the oscillation method. In recent years, it has been known that the size of a crystal grain formed in a semiconductor film becomes larger when using a CW laser oscillator such as an Ar laser or a YVO4 laser than when using a pulsed laser oscillator such as an excimer laser at the crystallization of the semiconductor film. When the size of the crystal grain in the semiconductor film becomes larger, the number of grain boundaries in a channel-forming region of a TFT formed with this semiconductor film decreases; therefore, the mobility increases. Accordingly, thus manufactured TFT can be used to develop a more sophisticated device. This is the reason why the CW laser is attracting attention.
Generally, when a silicon film having a thickness of several tens to several hundred nm usually used in a semiconductor device is crystallized with a CW YAG laser or YVO4 laser, a second harmonic having a shorter wavelength than the fundamental wave is used. This is because the second harmonic has higher absorption coefficient to the semiconductor film than the fundamental wave, which allows more effective crystallization of a silicon film. The fundamental wave is hardly employed in the step of crystallizing the silicon film by irradiating the silicon film with a laser beam.
As an example of this step, the following is given; a CW laser beam with a power of 10 W at the second harmonic (532 nm) is shaped into a linear spot having a length of 300 μm in the major-axis direction and 10 μm in the minor-axis direction and the beam spot is moved in the minor-axis direction to irradiate a semiconductor film. A region having large crystal grains that is obtained by one scanning has a width of approximately 200 μm (hereinafter the region having large crystal grains is referred to as a large crystal grain region). For this reason, in order to crystallize the whole surface of the substrate by laser irradiation, the laser irradiation needs to be conducted in such a way that the beam spot is displaced in the major-axis direction by the width of the large crystal grain region obtained by one scanning of the beam spot.
The invention in which a semiconductor film is irradiated with a laser beam shaped into a linear spot at an irradiation surface has been disclosed in Japanese Patent Application Laid-Open No.: 2003-257885